Michell Instruments QMA401, User Manual

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QMA401 User’s Manual
2 97450 Issue 2, March 2018
INTRODUCTION
1.2 Theory of Operation
The Quartz Crystal Microbalance (QCM) technology for moisture measurement is
based on monitoring the frequency of a hygroscopic-coated quartz crystal with specific
sensitivity to water vapor.
Bulk adsorption of water vapor onto the coated crystal causes an increase in the crystals
effective mass, modifying its oscillation frequency in a very precise and repeatable
manner. The frequency change is in proportion to the water vapor pressure in the
sample gas, providing a direct measurement of moisture content.
The sorption process is fully reversible with no long-term drift effect, giving a highly
reliable and repeatable measurement.
1.3 Sample Gas Path
The QMA401 measurement system must be supplied with gas at the required pressure
(to match that of its calibration) via the VCR connection on the rear of the instrument.
The flow is controlled automatically.
The sensor cell is located at the end of the sensor block and contains the sensor and
reference oscillators. Figure 1 shows a schematic diagram of this sampling system:
Sample In
Sample Out
DD
HE1 HE2
V1 V2 V3
MFC
QCH
PS
MG
FC1 FC2 FC3
Key
DD
Desiccant column
MG
Moisture generator
MFC
Mass flow controller
V1, V2, V3
Solenoid Valves
QCH
Sensor cell
HE1/HE2
Heat exchanger
PS
Pressure sensor
FC1
Flow control
Figure 1 Measurement System